Biuret manufacture

ABSTRACT

Methods are provided for recovering biuret from aqueous urea solutions and for producing relatively concentrated biuret-containing extracts which involve contacting a biuret-containing aqueous urea solution with the hydroxide ion form of an anion exchanger to retain at least a portion of the biuret on the exchanger, contacting the resulting biuret-containing ion exchanger with an aqueous extractant under conditions sufficient to form a biuret-containing extract, and contacting the resulting biuret-containing extract with the same or other biuret-containing anion exchanger in the same or subsequent cycles under conditions sufficient to increase the biuret concentration of the extract. The biuret-containing urea feed solution may also contain higher molecular weight urea condensation products, and the biuret can be selectively separated from such condensation products by the disclosed methods. Extracts containing about 1 weight percent biuret or more can be obtained from which biuret can be crystallized at temperatures above 0° C., and purified biuret of as much as 99.9 percent plus purity can be obtained from solutions containing higher molecular weight urea condensation products such as triuret, melamine, ammelide, and/or others. An integrated process also is provided which involves heating urea at a temperature sufficient to form biuret (in the presence or absence of higher molecular weight urea condensation products) and recovering biuret from an aqueous solution of the resulting pyrolyzed urea as described.

RELATED APPLICATIONS

This application is a continuation-in-part of our co-pending applications Ser. No. 567,271, filed Dec. 30, 1983 for Methods For Removing Biuret From Urea by Ion Exchange, Ser. No. 567,099, filed Dec. 30, 1983 for Ion Exchange Methods For Removing Biuret From Urea, Ser. No. 567,047, filed Dec. 30, 1983 for Methods For Removing Biuret From Urea, and Ser. No. 725,304, filed Apr. 19, 1985 for Methods for Purifying Biuret.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods for producing biuret, and in particular, it relates to methods for recovering biuret from urea.

2. Description of the Art

Biuret is widely used in commerce as a precursor for pharmaceuticals, herbicides, and other compounds, as an analytical reagent, and as a ruminant feed supplement. All of these utilities benefit from (if not require) the use of relatively pure biuret.

While biuret can be produced by several chemical methods, it is typically obtained by pyrolyzing urea at a temperature of at least 130° C. and for a period of time sufficient to convert at least a portion of the urea to biuret. An illustrative urea pyrolysis process is discussed by Shipley and Watchorn in British Patent No. 1,156,099. As disclosed by Shipley et al., the method produces a mixture of urea, biuret and higher molecular weight urea condensation products such as triuret, cyanuric acid, ammelide, melamine, ammonium cyanurate, methylene diurea, and/or other compounds.

Urea manufactured as solid prills is often treated at temperatures that result in some conversion of urea to biuret and, in many cases, the formation of higher molecular weight compounds as well. While the biuret concentration in prilled ureas is typically low, e.g., 0.5 to 3 weight percent, the amount of biuret contained in such products is substantial due to the large volume of prilled urea manufactured annually. Many of the commercial biuret-containing prilled ureas also contain higher molecular weight urea condensation products such as those mentioned above.

Many of the higher molecular weight condensation products present in some ureas appear to form by the reaction of urea with itself or with previously formed condensation products, or by reactions of, or between, previously formed condensation products. Others, such as methylene diurea, appear to form by the reaction of urea and/or condensation products with additives or other impurities such as formaldehyde which is sometimes employed as a urea anti-caking agent. Regardless of their origin, one or more of such impurities are known to exist in biuret obtained from urea by presently available methods as discussed by Shipley et al., supra, and Kaasenbrood in U.S. Pat. No. 3,185,731.

While urea pyrolysis and prilled urea manufacture afford an ample supply of biuret, the major utilities for biuret benefit from the use of that compound in relatively pure form. For instance, analytical procedures and pharmaceutical and herbicide manufacturing practices involving the use of biuret are most often unacceptably complicated by the presence of higher molecular weight condensation products, and the biuret dosage rate which can be employed in ruminant feed supplements is often limited by the toxicity of such impurities.

Methods presently available for commercially recovering biuret from urea typically involve low temperature crystallization procedures such as those discussed by Shipley et al. and Kaasenbrood, supra, in which the urea and/or biuret are recrystallized several times and separated, and the solid phase is washed to obtain purified urea and/or biuret. While such methods effectively separate biuret from urea, such separation is not complete, and the methods involve expensive, low temperature recrystallization procedures. A substantial amount of biuret generally remains in the urea fraction, and the biuret fraction typically contains minor amounts of urea, even after repeated recrystallization. Furthermore, such procedures do not efficiently separate biuret from urea and cogeneric impurities such as higher molecular weight urea condensation products. Typically, some or all of the higher molecular weight impurities remain in the biuret fraction.

Several authors have disclosed that biuret can be removed from urea by contacting an aqueous biuret-containing solution with the hydroxide ion form of an anion exchanger. For instance, Fuentes et al., U.S. Pat. No. 3,903,158, disclose that impurity biuret can be removed from aqueous urea solutions by ion exchange. Takahashi et al., "Determination of Biuret in Urea by Ion Exchange Resins," Soil and Plant Food, Vol. 3, No. 3, January 1958, pgs. 142-144, disclose that biuret can be separated from aqueous urea solutions and that the biuret retained on the ion exchanger can be eluted to obtain an indication of the biuret concentration in the urea feed.

While such processes are useful for purifying urea, their use for the recovery of biuret from aqueous urea solutions suffers from several disadvantages. The maximum biuret concentration which can be achieved in an ion exchanger regenerant, such as that employed by Fuentes et al. or Takahashi et al., is at most 0.5 weight percent and occurs only in the very initial stages of regeneration. The average biuret concentration in the total regenerant is typically well below 0.1 weight percent. This is because the volume of regenerant typically employed to restore the initial biuret-retaining ability of an ion exchanger, i.e., for regeneration, is so large that the biuret concentration in the total regenerant effluent is much lower than 0.1 weight percent. Almost without exception, it is preferable to completely regenerate ion exchangers to assure the greatest capability for removing the exchanged substance (in this case biuret) in subsequent cycles. Complete regeneration is more readily accomplished by the use of large regenerant volumes. In addition, the use of alkaline regenerants as disclosed by Fuentes et al. destroys biuret as disclosed in our U.S. Pat. No. 4,345,099 for Method of Selectively Removing Biuret from Urea and Compositions for Use Therein, the disclosure of which is incorporated herein by reference in its entirety.

The low biuret concentrations afforded in the exchanger regenerants of Fuentes et al. and Takahashi et al. render those solutions impractical for a variety of reasons. (In fact, the strongly alkaline regenerants of Fuentes et al. may not contain any biuret at all unless they are neutralized and/or cooled to prevent biuret hydrolysis.) The solubility of biuret in water at 0° C. is 0.53 weight percent. (Urea, Its Properties and Manufacture, Chao, Chao's Institute, West Covina, Calif., Library of Congress Catalogue Card No. Ai-11524.) Thus, biuret could not be crystallized from solutions of such low biuret concentration unless the solutions were modified to substantially depress their freezing points, and such modification of the solutions might itself prevent biuret crystallization even at lower temperatures. Thus, the commercial use of such solutions would require the shipment of large volumes of water with the attendant cost of such shipment, or evaporation of enough of the water to significantly increase biuret concentration. Obviously, such evaporation adds further expense to the process.

A further disadvantage associated with the prior art methods for separating biuret from urea involves the presence of a significant proportion of higher molecular weight urea condensation products in a substantial portion of biuret-containing ureas. While the prior art crystallization processes could be employed to separate biuret from some of the higher molecular weight urea condensation products, those processes, as mentioned above, require time-consuming, repeated low temperature recrystallization. The expense involved in such methods obviously increases the cost of biuret derived from such sources and limits its application as a result. For instance, ruminant feed supplement manufacturers generally choose to use relatively impure, less expensive biuret at dosage rates which are sufficiently low to avoid the toxic effects of the higher molecular weight impurities.

Furthermore, neither Fuentes et al. nor Takahashi et al. mention the presence of materials other than urea and biuret or the possibility that impurity-free biuret can be recovered from urea solutions which contain higher molecular weight urea condensation products. In fact, Takahashi et al. observe that "usually, urea for agriculture does not contain nitrogen compounds other than biuret." While that is often the case, some urea solutions, and in particular those formed from urea which has been pyrolyzed at temperatures above 130° C. for any significant period of time, contain a significant proportion of urea condensation products of higher molecular weight than biuret, some of which are toxic, and all of which can impair product utility.

SUMMARY OF THE INVENTION

It has now been discovered that biuret-containing extractants do not deactivate biuret-containing ion exchangers but effectively regenerate such exchangers and that the biuret concentration of a biuret-containing extractant recovered from an ion exchanger in one cycle can be increased by recycling the biuret-containing extractant into contact with one or more biuret-containing ion exchangers in one or more subsequent cycles. Without intending to be constrained by any particular theory, it presently appears that the extraction of biuret onto ion exchangers from solution and from ion exchangers into solutions is governed not by ion exchange equilibria but by the solubility of biuret in the extractant and the distribution coefficient of biuret between the extractant and ion exchanger. Thus, the biuret concentration of an unsaturated extractant can be increased by contact with an ion exchanger saturated with biuret.

More specifically, it has now been found that biuret-containing extractants recovered from the "regeneration" of biuret-containing ion exchangers can be repeatedly recycled into contact with biuret-containing ion exchangers in the same or subsequent cycles to increase the biuret concentration of the extractant to at least about 1 weight percent or more.

It is also been found that biuret can be selectively, efficiently and economically recovered from aqueous urea solutions containing biuret and higher molecular weight urea condensation products, and that the concentration of purified biuret in the extractant can be increased to commercially practicable levels by the recycle procedures described above.

The extractants can be either acidic, neutral or alkaline, and alkaline extractants are preferably neutralized and/or cooled immediately or shortly after extraction to minimize or eliminate biuret loss by hydrolysis. Higher biuret concentrations can be obtained in the extract by employing elevated extraction temperatures, longer contact times, e.g., lower extractant flow rates in column operations, and pure biuret can be obtained by chilling the concentrated extract to a temperature sufficient to crystallize biuret from the solution.

These methods eliminate the need for expensive, low temperature recrystallization processes employed by the prior art and enable the production of relatively concentrated biuret-containing extracts without diminishing exchanger efficiency for removing further quantities of biuret from additional portions of biuret-containing urea feed solution. They provide for the relatively inexpensive production of concentrated biuret-containing extracts (as compared to the cost of prior art processes), and they minimize or eliminate biuret loss with the urea solution or by hydrolysis.

These methods also provide for the efficient, selective and economical recovery of biuret from aqueous urea solutions containing biuret and higher molecular weight urea condensation products without the expense or complexity of repeated, low temperature crystallization (which is otherwise required to separate biuret from higher molecular weight urea condensation products). While the fate of the higher molecular weight impurities is not known with certainty, one or more of such impurities may be decomposed by contact with the anion exchanger while others may pass through the exchanger ahead of the biuret. Whatever the exact chemical mechanism, the methods of this invention can be employed to recover biuret of purity as high as 99.9 plus weight percent. Furthermore, these methods enable the recovery of purified biuret as a by-product of urea purification as described in our above identified co-pending application Ser. Nos. 567,271, 567,099, and 567,047, the disclosures of which are incorporated herein by reference in their entireties.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides methods for recovering biuret from aqueous urea solutions which involve contacting such aqueous solutions with the hydroxide ion form of an anion exchanger under conditions sufficient to retain at least a portion of the biuret on the exchanger and removing retained biuret from the exchanger with an aqueous extractant to form a biuret-containing extract which is then contacted with the same or another biuret-containing exchanger in the same or subsequent extraction cycles under conditions sufficient to increase the biuret concentration of the extract. These procedures are capable of producing relatively concentrated biuret-containing solutions without the necessity of evaporation and/or crystallization. Product solutions containing at least about 1 weight percent and even 10 weight percent more biuret can be obtained.

One specific embodiment of this invention involves selectively removing biuret from aqueous urea solutions which contain higher molecular weight urea condensation products by contacting such aqueous solutions with the described anion exchangers under conditions sufficient to retain at least a portion of the biuret on the exchanger and subsequently removing purified biuret from the exchanger by contact with an aqueous extractant. The resulting biuret-containing extract is recycled to increase biuret concentration as described above. This embodiment of the invention is capable of producing biuret of 99.9+percent purity from solutions containing substantial proportions of higher molecular weight urea condensation products such as triuret, melamine, etc.

Yet another embodiment involves an integrated process for producing biuret from urea in which urea is heated at a temperature of about 130° C. or higher for a period of time sufficient to convert at least a portion of the urea to biuret (in the presence or absence of higher molecular weight urea condensation products). The resulting mixture of urea and biuret is dissolved in water, and biuret is recovered as described above.

The aqueous biuret-containing urea solutions useful in this invention include, without limitation thereto, solutions recovered from any stage of the urea manufacturing process, solutions formed by dissolving urea, such as prilled urea, in water, and solutions formed by dissolving urea which has been heat treated to increase its biuret content. The solutions may also contain higher molecular weight urea condensation products such as triuret, cyanuric acid, ammonium cyanurate, methylene diurea, cyclic polyamides such as melamine and ammelide, and others. Biuret-containing urea solutions from which a portion of the biuret and/or urea have been separated by crystallization or otherwise as discussed, for instance, in British Patent No. 1,156,099 or U.S. Pat. No. 3,185,731, supra, may also be employed in these methods. Such solutions may also contain higher molecular weight urea condensation products. The disclosures of British Patent No. 1,156,099 and U.S. Pat. No. 3,185,731 are incorporated herein by reference in their entireties.

Many commercially available urea solids and solutions contain about 0.5 to about 3 weight percent biuret based on urea. Such ureas may also contain higher molecular weight urea condensation products. The higher molecular weight impurities typically constitute about 5, and generally at least about 10, and most often about 5 to about 50 weight percent of the composition based on biuret. Triuret, when present, usually accounts for approximately one-half of the impurity content of impure prilled ureas. Ureas which have been intentionally pyrolyzed, i.e., heated to form biuret, typically contain substantially higher proportions of biuret and higher molecular weight urea condensation products.

Pyrolysis is usually effected by treatment at a temperature of at least about 135° C., generally about 135° to about 180° C., for a period of time sufficient to convert at least a portion of the urea to biuret. Contact times of at least about 2 minutes are usually employed, although significant urea conversion usually requires treatment for about 5 minutes to about 5 hours. The rate of urea conversion increases with temperature; thus, the contact time required to achieve a certain degree of urea pyrolysis decreases as treatment temperature is increased.

While all of the urea can be pyrolyzed to biuret and higher molecular weight urea condensation products, higher conversions require the use of higher temperatures and/or longer contact times which result in higher impurity/biuret ratios and the loss of biuret product. Biuret loss under severe pyrolysis conditions is due, at least in part, to the pyrolysis of biuret to higher molecular weight compounds. Thus, the urea is typically only partially pyrolyzed under conditions sufficient to yield at least about 5 weight percent, generally about 10 to about 50 weight percent biuret based on urea. The higher biuret concentrations are usually associated with higher impurity/biuret ratios due to the more severe pyrolysis required to achieve such biuret levels. Further explanation of the relationship of pyrolysis time and temperature and of impurity concentration is given in "Urea, Its Properties and Manufacture, George C. Tsei-Yu-Chao, 1967, Library of Congress Catalog Card number Ai-11254, published by Chao's Institute, West Covina, Calif. particularly on pages 119-123, the disclosure of which is incorporated herein by reference.

Regardless of their source or the method of their manufacture, the useful biuret-containing urea feed solutions typically contain about 1 to about 80 weight percent, generally about 2 to about 70 weight percent solute, including urea and biuret in the presence or absence of higher molecular weight urea condensation products. Maximum solution concentration is usually determined by solution temperature which can range from 10° C. to about 100° C. However, at temperatures above 70° C. urea decomposition by hydrolysis becomes more rapid. Accordingly, aqueous solutions which contain substantial urea are preferably maintained at about 70° C. or less.

Urea has a solubility of approximately 80 weight percent in water at 70° C. and biuret will dissolve in water to a level of approximately 20 weight percent at the same temperature. However, biuret is more soluble in urea solutions. Thus, higher biuret concentrations can be achieved in the presence of substantial amounts of dissolved urea.

While the biuret-containing urea solutions can be employed as prepared, they usually contain sufficient ammonia to produce a relatively basic aqueous solution. Although this condition can be tolerated, pH levels of about 12 and above promote urea and biuret hydrolysis and are preferably avoided. Accordingly, the feed solutions will usually be relatively non-alkaline and will generally have a pH below 12, usually about 10 or less, preferably about 6 or less, and most preferably about 3 to about 6. Alkaline solutions can be neutralized and/or buffered to obtain the desired pH by adding any suitable organic or inorganic acid such as sulfuric, hydrochloric, nitric, acetic, etc., or buffering agents such as ammonium polyphosphate. Relatively acidic solutions are particularly preferred for use at higher temperatures and/or longer contact times.

The anion exchangers useful in the present invention can be either organic or inorganic, basic ion exchangers or combinations of organic and inorganic anion exchangers. The anion exchanger is preferably at least moderately basic and is most preferably a strongly basic anion exchanger such as those marketed by Rohm & Haas Company under the trademarks Amberlite IRA-400, IRA-458, and IRA-900, and the like, by the Dow Chemical Company under the Trademark Dowex I-X4, and by the BIORad Company under the trademark AG MP-1, and others.

The presently preferred anion exchangers are the strongly basic, organic ion exchange resins which contain tertiary and/or quaternary amine groups. Such anion exchangers can be prepared by the chloromethylation of a styrene-divinylbenzene copolymer which is then reacted with a tertiary or secondary amine, by the condensation of phenylenediamine with formaldehyde, or by the condensation of phenylenediamine, polyethyleneimine and formaldehyde. Particularly preferred anion exchangers are the quaternary amine types such as those disclosed in U.S. Pat. No. 2,591,573, the disclosure of which is incorporated herein by reference in its entirety.

The hydroxide ion form of the anion exchangers is presently preferred. However, many of the commercially available anion exchangers are manufactured and sold in other ionic forms, such as the chloride form, and require conversion to the hydroxide form prior to use. Such conversion can be readily accomplished by contacting the exchanger with an aqueous solution of a strong base as described, for instance, in "Ion Exchange with the Amberlite Resins" and "Amberlite IRA-400 - Laboratory Manual," both of which are available from the Resinous Products Division, Rohm & Haas Company, Washington Square, Philadelphia, Pennsylvania, and both of which are incorporated herein by reference in their entireties. Suitable procedures for converting the chloride ion form of anion exchangers to the preferred hydroxide ion form and for regenerating anion exchangers are also discussed in our above noted co-pending applications.

All of the process steps, including the extraction of biuret from the feed solution and the removal of biuret from the resulting biuret-containing anion exchanger, can be performed either by batch contacting or by continuous plug flow contacting in which the feed, extractant and regenerant solutions are passed through the anion exchanger retained in a relatively fixed bed. Plug flow systems can be operated either downflow or upflow, although downflow systems are generally preferred.

Each increment of the biuret-containing feed solution is usually contacted with the anion exchanger for at least about 30 seconds, preferably at least about one minute, most preferably at least about 5 minutes, and generally about one minute to about one hour. Contact times of about 5 to about 30 minutes are usually adequate to effect the desired degree of biuret removal. Such contact times correspond to flow rates of about 2 bed volumes per minute or less, usually about 1 bed volume per minute or less, preferably about 0.2 bed volumes per minute or less, and most preferably about 0.02 to about 1 bed volume per minute.

Contact of the anion exchanger with the biuret-containing feed is usually, although not necessarily, continued until the capacity of the anion exchanger is depleted. Depletion of exchanger capacity is indicated in the preferred, continuous, fixed bed systems by biuret breakthrough, which occurs when a detectable quantity of biuret is present in the solution recovered from the anion exchanger. However, the biuret sorption step can be continued past the point of biuret breakthrough if desired.

After completion of the biuret sorption step, the feed solution remaining in contact with the anion exchanger can be recovered and either returned to the feed solution reservoir, to a product accumulator, or otherwise as desired.

It is sometimes desirable, although not essential, to backwash the resin to remove foreign matter, to flush remaining feed solution from the anion exchanger, and/or to "reclassify" the bed of anion exchanger particles. Backwashing is usually effected by passing water rapidly upwardly through the bed to expand the bed by, e.g., 50 percent. However, substantial backwashing of the biuret-containing exchanger at this point in the operation is not preferred since, as discussed in our above noted co-pending applications, even neutral water is capable of removing biuret from the exchanger. Thus, biuret recovery can be maximized by deferring substantial backwashing until the biuret recovery step is completed as described hereinafter. However, the exchanger can be washed with a minor amount of water, e.g. one bed volume or less, and/or can be blown free of residual feed solution with a pressurized gas such as air, nitrogen, etc., to reduce or prevent contamination of the biuret product with the feed and other impurities.

The biuret-containing solution can be contacted with the anion exchanger at any temperature above the freezing point of the solution and below the upper temperature limit of the anion exchanger. Typically, the solution will be contacted with the exchanger at a temperature of about 0° to about 100° C., generally about 25° to about 70° C. and preferably about 30° to 70° C. While higher temperatures can be employed, urea decomposition rate (to CO₂ and ammonia) increases rapidly at temperatures above 70° C., and such temperatures are preferably avoided. Higher temperatures increase solubility and biuret sorption rate and thereby reduce the time required to sorb a given quantity of biuret. They also markedly increase urea and biuret hydrolysis rates under alkaline conditions. Therefore, it is preferable to acidify the feed solution at least to about neutrality and preferably to a pH below 7 when higher solution contacting temperatures are employed.

After the biuret sorption step, excess feed solution is removed from the exchanger, and the biuret-containing exchanger is contacted with an aqueous extractant under conditions sufficient to form an aqueous biuret-containing extract substantially free of higher molecular weight urea condensation products (when such products are present in the feed). The aqueous extractant can be contacted with the exchanger by either batch or column operations as described above with regard to the biuret sorption step.

The aqueous extractant can be either acidic, neutral or alkaline, although substantially nonalkaline extractants are presently preferred due to the tendency of alkaline media to hydrolyze biuret, particularly at elevated temperatures. Illustrative extractants include water, ammonium hydroxide solution, e.g.(1N NH₄ OH), and caustic, e.g., up to 8 percent sodium hydroxide. Calcium-free, carbonate-free, substantially nonalkaline extractants are presently preferred since alkaline materials promote biuret hydrolysis while calcium and carbonate ions impair exchanger regenerability for subsequent extraction of biuret from the feed solutions. Thus, deionized water or distilled water are presently preferred, and these can be acidified if desired.

At least slight acidification of the extractant, either before contacting with the exchanger or subsequent to the biuret removal step, is presently preferred, particularly when operating at higher temperatures. Acidification minimizes the loss of biuret by hydrolysis. Thus, the most preferred extractants usually have a pH of about 7 or less, preferably below about 6, and generally within the range of about 1 to about 7. Any organic or inorganic acid can be employed to effect the desired degree of acidification.

When alkaline extractants are employed, contacting temperature is preferably maintained below about 25° C., most preferably about 20° C. or less, and the extract is preferably neutralized with acid to approximately pH neutral or acidic conditions within a short time after its removal from contact with the exchanger.

The biuret-containing exchanger is contacted with a sufficient volume of extractant for a sufficient period of time to remove a substantial proportion of the biuret from the exchanger. Typically, at least one volume of extractant will be employed per volume of exchanger although much higher extractant volumes can be used. Thus, extractant volume will usually range from about 1 to about 100 volumes per volume of exchanger, although most operations will involve the use of about 1 to about 10 volumes of extractant per volume of exchanger.

In at least one respect, extractant volume is inversely proportional to biuret concentration in the product extract since biuret concentration is a function of total extract volume and the quantity of biuret removed from one or more ion exchangers by that volume of extractant. The quantity of biuret removed is a direct function of the biuret content of the ion exchanger and the quantity of ion exchanger contacted with a given volume of extractant. Thus, biuret concentration can be increased by either decreasing extractant volume or increasing the quantity of biuret-containing ion exchanger contacted with a fixed volume of extractant. As a general rule, each volume of extractant will be contacted with at least about 0.1, typically at least about 0.2 and preferably at least about 0.5 equivalent volume of ion exchanger which is initially saturated with biuret. Higher effective extractant/exchanger volume ratios can be achieved by recycling a given volume of extractant more than once and by reducing the total quantity of extractant employed to regenerate a given quantity of ion exchanger. In the latter instance, the volume of extractant passed over the ion exchanger during a given desorption cycle will typically be about 20 volumes or less, preferably about 18 volumes or less per volume of ion exchanger.

Shorter contact times are required to achieve the same degree of biuret removal at higher temperatures due to increased biuret solubility and high desorption rates. Thus, contact time can be varied depending on the temperature employed. Typically, contact times for the extraction step will be at least about 10 minutes to about 5 hours. Such contact times, in fixed bed systems, correspond to extractant flow rates of less than about 10, preferably less than about 5 volumes of extract per volume of exchanger per hour (V/V/hr.).

Extraction temperature should be sufficiently low to prevent substantial biuret hydrolysis, particularly when alkaline extractants are employed. Thus, extraction temperatures are generally below 40° C. and preferably below 30° C. when basic extractants are used. Biuret hydrolysis in alkaline systems is relatively slow at 24° C.

Neutral or acidic extractants can be employed at higher temperatures without significant biuret hydrolysis, and higher temperatures are presently preferred due to the higher solubility of biuret at such temperatures. For instance, biuret solubility in pure water is 0.53 weight percent at 0° C., 2 percent at 25° C., 7 percent at 50° C., 20 percent at 75° C., and almost 48 percent at 100° C. Elevated temperatures also increase biuret desorption rate from the exchanger. Accordingly, neutral or acidic extractants and elevated temperatures of at least about 25° C., generally about 25° to about 100° C., and preferably about 30° to about 100° C. are presently employed.

Extractant-exchanger contacting can be by either batch or co-current or countercurrent fixed bed procedures or combinations of these. Furthermore, single or multiple contacts with the same or different extractant can be employed in both batch and fixed bed systems. After a fresh quantity of extractant has been contacted with a biuret-containing ion exchanger, the resulting biuret-containing extract is recycled into contact either with the same ion exchanger or with another biuret-containing ion exchanger to increase the biuret concentration in the extract. Also, the first portion of extract (a fraction of an exchanger volume up to several volumes) recovered from the exchanger, which typically has a higher biuret content than subsequent portions of extract, can be recovered as product and/or processed by crystallization, evaporation, etc., and subsequent portions, typically of lower biuret content, can be recycled.

The extractant can be recycled up to 100 times or more depending on the biuret concentration desired, contact time between each volume of extractant during each cycle, temperature, and the quantity of biuret on each exchanger contacted. Usually, however, the extractant, or a portion thereof, will be recycled 1 to about 20 times unless biuret is recovered from the extractant. Biuret-containing extractant recovered after contact with a biuret-containing ion exchanger is typically recycled for a total of at least about three extraction cycles, preferably at least about five, and most preferably at least about ten extraction cycles. Biuret concentration can be increased each time the extractant is recycled until the extractant solubility level is reached at operating temperature. A fixed volume of extractant can be recycled indefinitely provided that biuret is recovered from the extractant by crystallization or otherwise.

Product extract will typically contain more than 0.5 weight percent, generally at least about 1 weight percent, and often about 2 to about 50 weight percent biuret. Biuret concentrations of at least about 4 weight percent, particularly at least about 10 weight percent, are preferred for the production of more concentrated solutions, e.g. by evaporation, or for the production of solid biuret by crystallization or otherwise. The higher biuret concentrations can be achieved by the use of longer contact times, lower volumes of extractant per volume of exchanger, higher exchanger biuret loadings, higher contacting temperatures, higher recycle ratios, or combinations of two or more of these procedures.

Following recovery of the biuret-containing extract from the ion exchanger, the exchanger can be employed to remove biuret from additional quantities of the biuret-containing feed solution with or without further regeneration. As discussed in our above-referenced copending applications, the useful ion exchangers can be regenerated by contact with water. However, the ion exchangers can lose a significant portion of their activity after multiple regenerations with only water and, in such instances, regeneration with caustic and/or acid is presently preferred. Regeneration with strong caustic, e.g. 2 to about 8 weight percent sodium hydroxide solution, is usually sufficient to restore most, if not all, of the anion exchanger activity. On occasion, however, the accumulation of impurities on the ion exchanger makes it desirable to regenerate with acids such as hydrochloric acid followed by reconversion of the ion exchanger to the useful hydroxide-ion form. Procedures suitable for effecting such regeneration are discussed in our above referenced copending applications.

Extracts prepared from urea solutions containing higher molecular weight urea condensation products in accordance with one embodiment of this invention are substantially free of the higher molecular weight urea condensation products present in the feed. Typically, the proportion of higher molecular weight impurities present in the extract will be less than one half, preferably less than one tenth, the concentration of those impurities in the feed solution based on biuret. Thus, biuret of greater than 95 percent purity can be obtained by these methods. When care is taken to avoid contamination of the extract with the residual feed solution on the ion exchanger, biuret purity of at least about 99 percent and even 99.9 percent plus can be achieved.

The recovered biuret-containing extract can be employed as is as a herbicide, chemical precursor, animal feed supplement, or for other utilities. Alternatively, a proportion or all of the water in the extract can be evaporated to obtain either crystalline biuret or a concentrated biuret solution. When elevated temperature evaporation is employed, care should be taken to assure that the extract is approximately pH neutral or acidic prior to exposure to elevated temperatures to avoid biuret loss.

In a particularly preferred embodiment, the biuret concentration in the extract is increased either by recycling or otherwise as described above or by evaporation to obtain a biuret concentration of at least about 4 weight percent, preferably at least about 10 weight percent biuret, after which the concentrated solution is chilled to a temperature, e.g. about 0° C., sufficient to crystallize biuret from the solution. The crystalline biuret can be recovered by any suitable solid-liquid separation means such as filtration, decanting, etc.

The invention is further described by the following examples which are illustrative of specific modes of practicing the invention and are not intended as limiting the scope of the invention as defined by the appended claims.

EXAMPLE 1

An aqueous solution containing approximately 50 weight percent urea, 1.9 weight percent biuret based on urea and a variety of higher molecular weight urea condensation products including methylene diurea and triuret was passed downwardly at a rate of 25 ml./min. through a glass ion exchange column packed with 250 ml. of the hydroxide ion form of BIORAD^(R) AG-MP1 at a temperature of approximately 25° C. until biuret was detected in the column effluent. Urea solution feed to the column was then discontinued.

The concentration of urea, biuret, and higher molecular weight urea condensation products in the urea feed solution and urea effluent from the ion exchange column were determined by liquid chromatographic analyses which demonstrated that the majority of the higher molecular weight urea condensation products had been removed from the biuret in the column effluent leaving purified biuret on the ion exchange resin. The purified biuret can be removed from the ion exchange resin by contacting the resin with water or alkaline or acidic solutions.

EXAMPLE 2

The aqueous urea solution described in Example 1 was passed downwardly through a 250 ml. bed of the hydroxide form of Rohm and Haas IRA-458 ion exchange resin at a rate of 0.3 bed volumes per minute until biuret was detected in the exchanger effluent. Urea feed to the column was then discontinued, residual urea feed solution was removed from the anion exchanger, and purified biuret was then extracted from the anion exchanger by passing downwardly through the column 10 bed volumes of a 4 weight percent sodium hydroxide solution in deionized water. The sodium hydroxide extractant was withdrawn from and recycled to an extractant reservoir having an initial total volume of 10 bed volumes (2500 ml).

The described biuret exchange and extraction steps were repeated seven additional cycles for a total of eight cycles. In each cycle, the same 10 volumes of sodium hydroxide extractant were employed to remove additional quantities of biuret from the anion exchanger. After five cycles, the biuret concentration in the sodium hydroxide extractant had reached 0.68 weight percent. Biuret concentration in the sodium hydroxide extractant increased to 1.1 weight percent by the end of the eighth cycle.

This operation demonstrates that the concentration of purified biuret in extracts can be increased by recycling the extract into contact with biuret-containing anion exchangers.

EXAMPLE 3

This Example illustrates one procedure by which the concentration of purified biuret in a non-alkaline extract can be increased by recycling the biuret-containing extract into contact with biuret-containing anion exchangers.

Purified biuret can be recovered from an aqueous solution containing 40 weight percent urea, 3 weight percent biuret based on urea, and 10 weight percent higher molecular weight urea condensation products based on biuret, by passing the solution downwardly over a packed bed of the hydroxide ion form of Rohm and Haas IRA-458 basic anion exchanger at a rate of 0.3 bed volumes per minute and a temperature of 40° C. until a substantial portion of biuret has been removed from the solution and retained on the resin. Urea feed solution flow is then discontinued, residual urea feed solution is removed from the anion exchange column and biuret is recovered from the anion exchanger by contact with 20 bed volumes of deionized water passed downwardly over the exchanger at a rate of 0.3 bed volumes per minute. The described biuret exchange and extraction steps can be repeated five times employing the same 20 bed volumes of deionized water extractant in each extraction step to obtain an extractant containing more than 0.5 weight percent biuret which is at least 95 percent pure biuret, i.e. which contains less than 5 weight percent higher molecular weight urea condensation products based on biuret.

EXAMPLE 4

The aqueous urea feed solution described in Example 1 was passed downwardly through a 200 ml. bed of the hydroxide ion form of Rohm and Haas basic anion exchange resin IRA-458 at a rate of 60 ml./min. until biuret was detected in the column effluent. Urea feed solution flow was then discontinued, residual urea feed solution was removed from the ion exchange resin, and the resin was washed with 20 bed volumes of deionized water passed downwardly over the ion exchanger at a rate of 60 ml. per minute. This treatment was sufficient to recover approximately 76 percent of the purified biuret from the anion exchanger. Maximum biuret concentration in the deionized water extract was approximately 0.5 weight percent which occurred in the first 2.5 bed volumes of deionized water extractant. Biuret concentration in subsequent portions of the extract gradually declined from that maximum level.

While particular embodiments of this invention have been described, it will be understood, of course, that the invention is not limited thereto since many obvious modifications can be made, and it is intended to include within this invention any such modifications as will fall within the spirit and scope of the appended claims. For instance, anion exchangers in which a portion or all of the exchangeable anions form conjugate acids which have first ionization constants (pK values) greater than that of biuret (pK=9.2) can be employed. Hence, ionic forms other than the hydroxide ion form can also be used. Illustrative of other ionic forms are berylliate, ascorbate, o-hydroxybenzoate, hypoiodate, silicilates, etc. 

We claim:
 1. A method for recovering biuret from an aqueous solution comprising urea and biuret which comprises (a) contacting said aqueous solution with the hydroxide ion form of an anion exchanger under conditions sufficient to retain at least a portion of said biuret on said anion exchanger, (b) contacting the resulting biuret-containing anion exchanger with an aqueous extractant under conditions sufficient to form a biuret-containing aqueous extract, and (c) contacting said biuret-containing extract with a biuret-containing anion exchanger under conditions sufficient to increase biuret concentration of said extract.
 2. The method defined in claim 1 wherein said aqueous extractant is contacted with a biuret-containing anion exchanger for a total of at least three cycles and under conditions sufficient to increase the biuret content of said extract to at least about 1 weight percent.
 3. The method defined in claim 1 wherein said aqueous extractant is contacted with a biuret-containing anion exchanger for at least about three cycles sufficient to increase the biuret concentration of said extract to at least about 4 weight percent, and, during at least one of said cycles, said extractant is contacted with a biuret-containing anion exchanger at a temperature of at least about 30° C.
 4. The method defined in claim 1 wherein said aqueous extractant contacted with said biuret-containing anion exchanger has a pH below about
 7. 5. The method defined in claim 4 wherein said pH is below about
 6. 6. The method defined in claim 1 which further comprises the step of reducing the pH of said biuret-containing extract recovered from said anion exchanger.
 7. The method defined in claim 1 wherein said aqueous extractant is contacted with said biuret-containing anion exchanger at a temperature of at least about 30° C.
 8. The method defined in claim 1 wherein said aqueous extractant is contacted with said biuret-containing anion exchanger at a temperature of at least about 50° C.
 9. The method defined in claim 1 wherein said aqueous extractant has a pH below 7 and is contacted with said biuret-containing anion exchanger at a temperature of at least about 30° C.
 10. The method defined in claim 2 wherein at least a portion of said biuret is recovered from said extract.
 11. The method defined in claim 2 wherein said biuret-containing extract is cooled to a temperature sufficient to crystallize at least a portion of said biuret from said extract.
 12. The method defined in claim 1 wherein said aqueous urea solution comprises at least about 5 weight percent biuret.
 13. The method defined in claim 1 wherein said aqueous solution comprises at least about 10 weight percent urea and at least about 10 weight percent biuret based on said urea, and said solution is prepared, at least in part, by heating urea to a temperature of at least about 135° C. for a period of time sufficient to convert at least a portion of said urea to biuret.
 14. The method defined in claim 1 wherein said biuret-containing extract is recycled into contact with one or more biuret-containing anion exchangers for a number of cycles sufficient to increase the biuret concentration of said extract to a level above the biuret solubility limit at 0° C., and the resulting extract is cooled to a temperature sufficient to crystallize at least a portion of said biuret from said extract.
 15. The method defined in claim 1 wherein said anion exchanger is retained in a relatively fixed bed, and said extractant is contacted with said anion exchanger by passing the same through said bed at a rate corresponding to less than about 10 volumes of said extractant per volume of said anion exchanger per hour.
 16. The method defined in claim 15 wherein said extractant is water having a pH of about 6 to about
 8. 17. The method defined in claim 15 wherein said extractant and said solution are contacted with said anion exchanger by passing the same downwardly over said relatively fixed bed.
 18. The method defined in claim 17 wherein the total volume of said extractant and said extract contacted with said anion exchanger in each of steps (b) and (c) corresponds to about 20 bed-volumes or less.
 19. The method defined in claim 17 wherein the total volume of said extractant and said extract contacted with said anion exchanger in each of steps (b) and (c) corresponds to about 15 bed-volumes or less, and, in at least one cycle, said biuret-containing extract is contacted with said biuret-containing anion exchanger in step (c) at a temperature of at least about 30° C.
 20. A method for recovering biuret from an aqueous solution comprising urea and biuret which comprises (a) contacting said aqueous solution with the hydroxide ion form of an anion exchanger under conditions sufficient to retain at least a portion of said biuret on said anion exchanger, (b) contacting the resulting biuret-containing anion exchanger with an aqueous extractant under conditions sufficient to extract at least a portion of said biuret from said ion exchanger, and (c) recycling at least a portion of the resulting biuret-containing extract into contact with a biuret-containing ion exchanger for a total of at least three cycles and under conditions sufficient to increase the biuret content of said extract to at least about 1 weight percent, wherein said anion exchanger is retained in a relatively fixed bed, and said aqueous solution, said extractant, and said biuret-containing extract are contacted with said anion exchanger by passing the same through said relatively fixed bed.
 21. The method defined in claim 20 wherein said aqueous extract is contacted with a biuret-containing ion exchanger for a total of at least about five cycles and under conditions sufficient to increase the biuret content of said extractant to at least about 2 weight percent.
 22. The method defined in claim 20 wherein said aqueous extractant is contacted with said biuret-containing anion exchanger in at least one cycle at a temperature of at least about 50° C. and under conditions sufficient to produce an extract containing at least about 4 weight percent biuret.
 23. The method defined in claim 20 wherein said aqueous extractant is contacted with a biuret-containing anion exchanger at a pH of about 7 or less and a temperature of at least about 50° C. for a total of at least about three cycles sufficient to increase the biuret content of said extract to at least about 10 weight percent.
 24. The method defined in claim 23 which further comprises the step of recovering at least a portion of the biuret from said extract, and forming an extract of reduced biuret content.
 25. The method defined in claim 24 wherein said extract of reduced biuret content is contacted with a biuret-containing ion exchanger under conditions sufficient to remove at least a portion of said biuret from said ion exchanger.
 26. The method defined in claim 23 which further comprises the step of cooling said extract to a temperature sufficient to crystallize at least a portion of said biuret from said extract.
 27. The method defined in claim 20 wherein said aqueous extract and said extractant are contacted with said biuret-containing anion exchanger at rates corresponding to about 10 volumes per volume of said ion exchanger per hour or less.
 28. The method defined in claim 20 wherein an initial portion of said extractant of relatively high biuret content recovered from said anion exchanger is separated from subsequent portions of said extractant of relatively lower biuret content recovered from said biuret-containing anion exchanger, and said subsequent portions are recycled into contact with at least one biuret-containing anion exchanger.
 29. The method defined in claim 20 wherein at least a portion of said biuret-containing extract is recycled into contact with said biuret-containing anion exchanger in the same cycle.
 30. The method defined in claim 28 wherein said subsequent portion of said extractant of relatively low biuret content is recycled into contact with said biuret-containing anion exchanger in the same cycle.
 31. The method defined in claim 20 wherein said biuret-containing ion exchanger is contacted with about 15 volumes or less of a member selected from the group consisting of said extractant, said biuret-containing extract, and combinations thereof per volume of said anion exchanger in at least one of said cycles.
 32. A method for producing biuret which comprises (a) heating urea at a temperature sufficient to convert a portion of said urea to biuret and form a mixture comprising urea and at least about 5 weight percent biuret, (b) contacting an aqueous solution comprising said mixture with the hydroxide ion form of an anion exchanger under conditions sufficient to retain at least a portion of said biuret on said anion exchanger, (c) contacting the resulting biuret-containing anion exchanger with an aqueous extractant under conditions sufficient to form a biuret-containing extract, and (d) recycling at least a portion of said biuret-containing extract into contact with a biuret-containing anion exchanger for a total of at least three cycles.
 33. The method defined in claim 32 wherein said urea is heated at a temperature of at least about 135° C., and said biuret-containing extract has a pH of about 7 or less.
 34. The method defined in claim 32 wherein said aqueous extractant has a pH of about 7 or less and is contacted with a biuret-containing anion exchanger for a total of at least about five cycles, during at least one of said cycles said extract is contacted with said anion exchanger at a temperature of at least about 30° C. and under conditions sufficient to increase the biuret concentration of said extract to at least about 2 weight percent. 